1. Field of the Invention
This invention relates to a sensor for eddy current testing for locating of surface or near-surface flaws in electrically conductive materials.
2. Description of the Related Art
Use of eddy currents in nondestructive testing of specimens has been known from late in the 19th century. Increased research on nondestructive testing has been motivated by the need for precise evaluation of cracks and flaws for the assessment of the expected life of mechanical components. Methods for such testing for detection of defects such as fatigue cracks, inclusions, voids, and the like include dye penetrants, x-ray and ultrasonic testing, and eddy current testing (ECT) in conductive materials.
Many different methods are known for eddy current testing to locate surface or near-surface flaws in electrically conductive materials. Generally, the methods are based on inducing small circular electrical currents (eddy currents) in metallic materials using a coil excited by an alternating current. When appropriately arranged, the excitation magnetic field in the near surface region is perpendicular to the surface, thereby inducing circular currents in the plane of the surface. The disruption of the eddy currents by a discontinuity is similar to the disruption of magnetic fields, except that a wider variety of discontinuities and physical properties affect eddy currents, such as changes in grain size; surface treatment (especially heat treatment); coating thickness; hardness; integrity caused by discontinuities such as cracks, inclusions, dents and holes; dimensions such as thickness, eccentricity, diameter, or separation distance; and alloy composition.
A typical eddy current technique that is known in the art utilizes a coil placed close to the area to be inspected to which an alternating current of 50-500,000 Hz is applied. Changes in the specimen that affect the generation of eddy currents are detected as a change in the impedance (Z, the ratio of coil voltage to coil current) of the coil and are sensed through phase changes in the voltages or currents in the exciting or sensing coils. This change in the impedance may be seen in the deflection of a meter, an oscilloscope presentation, a strip chart recording, lights or alarm actuation, digital readouts or operation of a manufacturing process.
Eddy current testing as now known utilizes any of a large variety of coil configurations depending on one or more of the following: the electrical characteristics and constituents of the component being investigated, the method of detection, the defect information that is desired, and the geometry of the part to be inspected. The field of the current in the specimen being investigated is set up so that it opposes the field producing the current. Two coils may be used in which one coil is used to induce the eddy currents in the specimen (the primary coil), and the second coil is used to sense the eddy current field (secondary coil). Often the sensing coil is smaller, closer to the specimen, and oriented differently than the exciting coil. If the two coils are wound so that their fields oppose each other, the resulting differential coil allows absolute measurement such as single impedance measurement of the two windings as a single coil. A signal is produced only when the specimen near one winding is different from the specimen near the other coil. Such configurations are generally not usable for thickness measurements or for gradual changes in the specimen, but localized corrosion pits and small cracks are well-defined by differential coils.
Alternatively, the magnetic field and its perturbation by discontinuities in the specimen can be detected by Hall effect transducers in place of a secondary coil. One Hall effect transducer can provide a very localized sensing element or an array of sensing elements can be used for examination of small or discrete areas of a specimen.
Many of the prior methods for sensing eddy currents are complicated, are not sufficiently accurate, have insufficient spatial resolution, are bulky, are not sensitive to low fields, are not self-rectifying or directional, or have other limitations. Generally, prior methods of detection of eddy currents use detection in a perpendicular direction to the surface being analyzed. Tangential detection is reported in U.S. Pat. No. 5,864,229, where the detection coils are oriented in parallel direction to the surface (referred to as xe2x80x9ccurrent perturbation coilsxe2x80x9d), and are bulky and do not allow a very localized detection of the field. Also, the probes using these coils cannot be miniaturized or planar integrated on silicon.
Patents for prior eddy current probe testing systems include U.S. Pat. Nos. 5,019,777; 5,068,608; 5,483,160; and 5,864,229. The disclosure of these patents and all other patents referred to herein is incorporated herein by reference.
The invention herein is based on the giant magnetoresistive phenomenon, a recently discovered effect found in metallic thin films consisting of magnetic layers separated by thin nonmagnetic layers a few nanometers thick. Researchers observed a large decrease in the resistance by applying a magnetic field to the films. The cause of this effect is the spin dependence of electron scattering and the spin polarization of conduction electrons in ferromagnetic metals. The effect depends on the relative orientation of the magnetization in the adjacent ferromagnetic layers. If the orientation is parallel, there is minimum electrical resistance of the structure and when the orientation is antiparallel, there is maximum electrical resistance of the structure. When no external field is applied, the adjacent ferromagnetic layers have antiparallel orientation, due to antiferromagnetic coupling of these layers when separated by a nonmagnetic layer (usually copper) of a defined thickness. The initial orientation of the magnetization in the ferromagnetic layers is obtained by either depositing the ferromagnetic layers under the influence of a magnetic fields, which gives the orientation of the magnetization, or by depositing narrow strips of giant magnetoresistive (GMR) films that automatically orient the magnetization (magnetic domains) along the main dimension of the strips, due to the shape-induced anisotropy. When an external field is applied within the plane of the films, the magnetization of all the ferromagnetic layers tends to orient along the direction of this applied field, resulting in the parallel configuration described above, and consequently, in a lower resistance of the structure.
GMR devices (U.S. Pat. Nos. 5,595,830; 5,569,544; and 5,617,071 of Daughton) are used as magnetic field sensors and have been used for measuring displacement, angular position/speed measurement, current measurement, magnetic media detection, magnetic memory, earth""s field detection and for biosensors. Although others have used a GMR sensor to detect the vertical (perpendicular to the specimen surface) component of the magnetic field to detect deep buried flaws in aluminum multilayered structures, others have not previously used a GMR sensor to detect fields in a direction parallel to the specimen surface (see Wincheski et al., Review of Progress in QNDE 18: 1177, 1999 and Ward et al., Review of Progress in QNDE 17: 291-298, 1998).
The sensor of the invention herein can be placed closer to the surface of the specimen to be tested than previous sensors, and therefore the device has a higher sensitivity by reducing sensor lift-off. The coil diameter of the invention is greatly reduced in size, which is not physically possible with the previous vertical sensors, and therefore the invention has much better spatial resolution using flat coils on top of the sensor. The integration of the coil and sensor in the invention using planar technology is thus possible in the invention while not being possible in the vertical sensors. With the invention, detection of cracks near edges is possible by eliminating the large signal produced by the edge, using a proper orientation of the sensor""s axis, parallel to the edge, which is not possible in the vertical sensor.
It is therefore an object of the invention to provide a simple, small, sensitive uni-polar eddy current sensor that is able to accurately detect axial and circumferential flaws in conductive materials, provide information on cracks, and enhance the spatial resolution of eddy current testing and detect cracks located near the sharp edges.
It is a further object of the invention to provide an eddy current sensor with a simplified signal conditioning circuit that does not require synchronous detection, due to its being a uni-polar device.
It is a further object of the invention to provide an eddy current sensor that has directional characteristics due to the magnetic easy-axis produced by poling the films during deposition.
It is a further object of the invention to provide an eddy current sensor which is used to detect fields coplanar to the specimen surface and in a specific direction within this plane. The flat coils of this sensor can be placed closer to the specimen surface and is very sensitive.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.
The invention herein is a giant magnetoresistive (GMR) sensor combined with a circular coil to create an eddy current probe, which is extremely sensitive and able to detect magnetic fields of the order of 10xe2x88x925 T. The sensitive axis of the GMR sensor is oriented parallel to the specimen surface.
Other objects and features of the inventions will be more fully apparent from the following disclosure and appended claims.